Hello, I made this userscript long ago but I never made a thread about it.
This userscript will automatically log you in whenever you're visiting e621.net and you're not logged in.

+ REQUIRES AN USERSCRIPT EXTENSION +

How to install:
1- Click the installation link
2- Click "Install"
3- Put in your log in credentials (http://i.imgur.com/XoaryTT.png)

Installation link:
https://gist.github.com/anonymous/f9b4382092ec16da19d9/raw/eccf534f30700c02b5b3714d8ba777d30d8437a1/e621-autologin-v1.user.js

TESTED ON:
+ Google Chrome 42.0.2311.152 with Tampermonkey 3.10.109
+ Firefox 31.6.0 with Greasemonkey 2.3

@aurel
In some cases, cookies don't last long. Incognito, Tor Browser, ...

I use e6 incognito a lot, this will be helpful :D

aurel said:
uhh.. but every browser autologins automatically.

*checks code suspiciously*
nothing really wrong, but i would be worried that greasemonkey sends ALL the code somewhere (in case something goes wrong).

Might be helpful if you don't accept cookies? Dunno.

I personally wouldn't advise storing logins in plaintext like that (it's just a bad habit to get into) as it can be a security risk in and of itself. Not that I have reason to believe that OP is planning something malicious, but just to be safe I would recommend resisting the urge to install any updates that come through (I don't remember if tampermonkey/greasemonkey does a check on its own, but the script itself doesn't), at least until someone can look at the updated code. As with other e6-related tools, if anybody has reason to believe there's anything malicious going on, notify me or one of the other admins as soon as possible so that we can take a look at it and hopefully prevent others from being affected by it.

But for now the code looks I see looks fine as far as I can tell.

Tokaido said:
I use e6 incognito a lot, this will be helpful :D

LeAwkwardGhost said:
@aurel
In some cases, cookies don't last long. Incognito, Tor Browser, ...

Edit: Oh right, I forgot about incognito.

There's no update URL set, so the script won't update itself (the credentials would be lost if it did). And for the password storage, I couldn't find a secure way to store them.